U.S. patent number 4,841,160 [Application Number 07/278,636] was granted by the patent office on 1989-06-20 for power supply switching circuit.
This patent grant is currently assigned to NCR Corporation. Invention is credited to Luis A. Diaz, Steven A. Yon.
United States Patent |
4,841,160 |
Yon , et al. |
June 20, 1989 |
Power supply switching circuit
Abstract
The subject invention is a power supply having an input for
receiving a power signal, a DC storage device connected to the
input, a primary output connected to the DC storage device and
connectable to a battery, a DC to DC converter for converting the
voltage at the DC storage device to an auxiliary DC voltage, the
converter having a supply voltage input, and a switching circuit.
The switching circuit includes first, second and third switches.
The first switch is responsive to the power signal for providing
the DC storage device voltage to the converter supply voltage
input. The second switch is responsive to the voltage on the
battery for providing the DC storage device voltage to the
converter supply voltage input. The third switch is responsive to
an auxiliary voltage on/off signal for overriding the second
swtich.
Inventors: |
Yon; Steven A. (Altamonte
Springs, FL), Diaz; Luis A. (Apopka, FL) |
Assignee: |
NCR Corporation (Dayton,
OH)
|
Family
ID: |
23065751 |
Appl.
No.: |
07/278,636 |
Filed: |
December 1, 1988 |
Current U.S.
Class: |
307/66; 307/64;
307/87; 363/97; 365/229; 363/41; 365/228 |
Current CPC
Class: |
H02J
9/062 (20130101) |
Current International
Class: |
H02J
9/06 (20060101); H02J 007/00 () |
Field of
Search: |
;307/64-87
;363/41,15,26,97 ;365/226-229 ;320/14 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Shoop, Jr.; William M.
Assistant Examiner: Ip; Paul
Attorney, Agent or Firm: Hawk, Jr.; Wilbert Jewett; Stephen
F. Foote; Douglas S.
Claims
What is desired to be secured by Letters Patent of the United
States is as follows:
1. A power supply comprising:
an input for receiving a power signal;
a DC storage device connected to said input;
a voltage regulator connected to said DC storage device and having
a primary output connectable to a battery;
a first diode connecting said primary output to said DC storage
device;
a DC to DC converter for converting the voltage at said DC storage
device to an auxiliary DC voltage, said converter having a supply
voltage input; and
a switching circuit including:
a first switch responsive to said power signal for providing said
DC storage device voltage to said converter supply voltage
input;
a second switch responsive to the voltage on said battery for
providing said DC storage device voltage to said converter supply
voltage input; and
a third switch responsive to an auxiliary voltage on/off signal for
overriding said second switch.
2. The power supply of claim 1 wherein said first switch comprises
first and second transistors, said first transistor connecting the
control electrode of said second transistor to a reference
potential terminal, the control electrode of said first transistor
connected between said power supply input and said reference
potential terminal, and said second transistor being connected
between said DC storage device and said converter supply voltage
input.
3. The power supply of claim 1 wherein said second switch comprises
second and third transistors, said third transistor connecting the
control electrode of said second transistor to a reference
potential terminal, the control electrode of said third transistor
connected between said primary output and said reference potential
terminal, and said second transistor being connected between said
DC storage device and said converter supply voltage input.
4. The power supply of claim 3 wherein said third switch comprises
a fourth transistor connected between the control electrode of said
third transistor and said reference potential terminal, the control
electrode of said fourth transistor receiving said on/off
signal.
5. The power supply of claim 4 further comprising:
a first capacitor an first resistor connected in parallel between
said fourth transistor control electrode and said reference
potential terminal;
an input terminal for receiving said on/off signal; and
a second resistor connected between said input terminal and the
control electrode of said fourth transistor.
6. The power supply of claim 5 wherein said fourth transistor is a
field effect transistor with its drain connected to the control
electrode of said third transistor, its source connected to said
reference potential terminal and its gate connected to said second
resistor.
7. The power supply of claim 3 further comprising:
a third resistor and zener diode series connected between the
control electrode of said third transistor and said primary
output.
8. The power supply of claim 7 wherein the breakdown voltage of
said zener diode is equal to a predetermined low voltage limit on
said battery.
9. The power supply of claim 7 wherein said third resistor is a
variable resistor.
10. The power supply of claim 3 further comprising:
a fourth resistor connected between the control electrode of said
third transistor and said reference potential terminal.
11. The power supply of claim 10 wherein said second transistor has
emitter, collector and base electrodes with said emitter electrode
connected to said DC storage device, said collector electrode
connected to said converter supply voltage input and said base
electrode connected to said first transistor, said power supply
further comprising:
a tenth resistor connected between the control electrode of said
third transistor and the collector electrode of said second
transistor.
12. The power supply of claim 2 further comprising:
a fifth resistor connected between said first transistor and the
control electrode of said second transistor.
13. The power supply of claim 12 further comprising:
a sixth resistor connected between the control electrode of said
second transistor and said DC storage device.
14. The power supply of claim 13 wherein said first transistor has
emitter, collector and base electrodes with said emitter electrode
connected to said reference potential terminal and said collector
electrode connected to said fifth resistor.
15. The power supply of claim 14 further comprising:
a seventh resistor connected between said base electrode and said
reference potential terminal; and
an eighth resistor and second capacitor, said eighth resistor
connected between said base electrode and said second capacitor,
and said second capacitor connected between said eighth resistor
and said reference potential terminal.
16. The power supply of claim 15 further comprising a second diode
and ninth resistor series connected between said power supply input
and the connection between said eighth resistor and second
capacitor.
17. The power supply of claim 3 further comprising:
a fifth resistor connected between said third transistor and the
control electrode of said second transistor.
18. The power supply of claim 17 wherein said third transistor has
emitter, collector and base electrodes with said emitter electrode
connected to said reference potential terminal and said collector
electrode connected to said fifth resistor.
19. The power supply of claim 1 wherein said DC to DC converter is
a down-switcher.
20. The power supply of claim 1 further comprising:
a third diode connected between said power signal input and said DC
storage device.
21. The power supply of claim 1 wherein said DC storage device is a
capacitor.
22. The power supply of claim 3 further comprising:
a fourth diode connected between said second transistor and said
converter supply voltage input.
23. A power supply comprising:
an input for receiving a power signal;
a DC storage device connected to said input;
a voltage regulator connected to said DC storage device and having
a primary output connectable to a battery;
a first diode connecting said primary output to said DC storage
device;
a DC to DC converter for converting the voltage at said DC storage
device to an auxiliary DC voltage, said converter having a supply
voltage input; and
a switching circuit including:
a first switch having a first and a second transistor, said first
transistor connecting the control electrode of said second
transistor to a reference potential terminal, the control electrode
of said first transistor connected between said power supply input
and said reference potential terminal, and said second transistor
being connected between said DC storage device and said converter
supply voltage input;
a second switch having said second and a third transistor, said
third transistor connecting the control electrode of said second
transistor to a reference potential terminal, the control electrode
of said third transistor connected between said primary output and
said reference potential terminal; and
a third switch responsive to an auxiliary voltage on/off signal for
overriding said second switch, said third switch having a fourth
transistor connected between the control electrode of said third
transistor and said reference potential terminal, the control
electrode of said fourth transistor receiving said on/off
signal.
24. The power supply of claim 23 further comprising:
a third resistor and zener diode series connected between the
control electrode of said third transistor and said primary output,
wherein the breakdown voltage of said zener diode is equal to a
predetermined low voltage limit on said battery and said third
resistor is a variable resistor.
25. The power supply of claim 24 further comprising:
a fifth resistor connected between said first transistor and the
control electrode of said second transistor; and
a sixth resistor connected between the control electrode of said
second transistor and said DC storage device.
26. The power supply of claim 25 wherein said second transistor has
emitter, collector and base electrodes with said emitter electrode
connected to said DC storage device, said collector electrode
connected to said converter supply voltage input and said base
electrode connected to said first transistor, said power supply
further comprising:
a tenth resistor connected between the control electrode of said
third transistor and the collector electrode of said second
transistor.
27. The power supply of claim 26 further comprising:
a fourth diode connected between the collector of said second
transistor and said converter supply voltage input.
28. The power supply of claim 27 wherein said first transistor has
emitter, collector and base electrodes with said emitter electrode
connected to said reference potential terminal and said collector
electrode connected to said fifth resistor, and wherein said third
transistor has emitter, collector and base electrodes with said
emitter electrode connected to said reference potential terminal
and said collector electrode connected to said fifth resistor.
29. The power supply of claim 28 further comprising:
a fourth resistor connected between the base electrode of said
third transistor and said reference potential terminal.
30. The power supply of claim 29 further comprising:
a seventh resistor connected between the base electrode of said
first transistor and said reference potential terminal;
an eighth resistor and second capacitor, said eighth resistor
connected between the base electrode of said first transistor and
said second capacitor, and said second capacitor connected between
said eighth resistor and said reference potential terminal; and
a second diode and ninth resistor series connected between said
power signal input and the connection between said eighth resistor
and second capacitor.
31. The power supply of claim 30 wherein said fourth transistor is
a field effect transistor with its drain connected to the base
electrode of said third transistor and its source connected to said
reference potential terminal, said power supply further
comprising:
a first capacitor and first resistor connected in parallel between
the gate of said fourth transistor and said reference potential
terminal;
an input terminal for receiving said on/off signal; and
a second resistor connected between said input terminal and the
gate of said fourth transistor.
32. The power supply of claim 31 wherein said DC storage device is
a third capacitor, and wherein said power supply further
comprises:
a third diode connected between said power signal input and said
third capacitor.
33. The power supply of claim 32 wherein said DC to DC converter is
a down-switcher.
34. A power supply comprising:
an input for receiving a power signal;
a DC storage device connected to said input;
a primary output connected to said DC storage device and
connectable to a battery;
a DC to DC converter for converting the voltage at said DC storage
device to an auxiliary DC voltage, said converter having a supply
voltage input; and
a switching circuit including:
a first switch responsive to said power signal for providing said
DC storage device voltage to said converter supply voltage input;
and
a second switch responsive to the voltage on said battery for
providing said DC storage device voltage to said converter supply
voltage input.
35. The power supply of claim 34 wherein said first switch
comprises first and second transistors, said first transistor
connecting the control electrode of said second transistor to a
reference potential terminal, the control electrode of said first
transistor connected between said power power supply input and said
reference potential terminal, and said second transistor being
connected between said DC storage device and said converter supply
voltage input.
36. The power supply of claim 34 wherein said second switch
comprises second and third transistors, said third transistor
connecting the control electrode of said second transistor to a
reference potential terminal, the control electrode of said third
transistor connected between said primary output and said reference
potential terminal, and said second transistor being connected
between said DC storage device and said converter supply voltage
input.
37. The power supply of claim 34 wherein said switching circuit
further comprises:
a third switch responsive to an auxiliary voltage on/off signal for
overriding said second switch.
38. The power supply of claim 37 wherein said third switch
comprises a fourth transistor connected between the control
electrode of said third transistor and said reference potential
terminal, the control electrode of said fourth transistor receiving
said on/off signal.
Description
The present invention relates to a power supply which provides
primary and auxiliary DC voltage to electronic equipment. More
particularly, it relates to a switching circuit for controlling the
supply of auxiliary DC voltage.
BACKGROUND OF THE INVENTION
Most electronic and computer systems require DC voltage for their
operation. Many systems further require more than one DC voltage
level. A typical system has a power supply which connects to a
standard 110 volt or 220 volt AC line and then converts the AC to
one or more DC voltage levels. In order to ensure an uninterrupted
supply of power, some systems employ a battery at the primary DC
output of the power supply to hold up its level in the event of a
loss of AC power. Secondary or auxiliary DC outputs of the power
supply do not normally have a battery to hold up their level in the
event of a power interruption but receive power from the battery at
the primary DC output when AC power fails.
An auxiliary DC output voltage may be provided by a DC to DC
converter which may be either an up-switcher or down-switcher
circuit depending upon the level of voltage desired at the
auxiliary output. Many converters have active electronic elements
which require a supply voltage to operate. For example, a converter
may employ a pulse width modulator chip which requires a minimum
supply voltage to operate.
It is desirable to provide a supply voltage to the converter
whenever the power supply is receiving AC power. Furthermore, it is
desirable to maintain this supply voltage in the event of an
interruption of AC power. However, maintenance of a converter
supply voltage must stop if excessive battery discharge would
result. Other conditions where it is desirable to discontinue
providing supply voltage to the converter when AC power is not
available include an improperly installed battery at the DC output
and receipt of a control signal from the system.
OBJECTS OF THE INVENTION
It is therefore an object of the present invention to provide a new
and improved power supply.
It is another object of the present invention to provide a power
supply with a DC to DC converter, having a new and improved
switching circuit for providing supply voltage to the
converter.
It is a further object of the present invention to provide a
switching circuit for a power supply which provides supply voltage
to a DC to DC converter whenever AC power is available, but
provides supply voltage when AC power is not available only when a
storage battery is not unduly discharged, the battery is not
installed backwards, and a remote shut-off signal has not been
received.
SUMMARY OF THE INVENTION
The present invention is a power supply comprising an input for
receiving a power signal, a DC storage device connected to the
input, a primary output connected to the DC storage device and
connectable to a battery, a DC to DC converter for converting the
voltage at the DC storage device to an auxiliary DC voltage, the
converter having a supply voltage input, and a switching circuit.
The switching circuit includes a first switch responsive to the
power signal for providing the DC storage device voltage to the
converter supply voltage input. The switching circuit also includes
a second switch responsive to the voltage on the battery for
providing the DC storage device voltage to the converter supply
voltage input.
According to another embodiment, the present invention includes a
voltage regulator connected to the DC storage device and a first
diode connecting the primary output to the DC storage device. A
further form of the invention includes a third switch responsive to
an auxiliary voltage on/off signal for overriding the second
switch.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 show a circuit diagram of a power supply according to
one form of the present invention.
DETAIL DESCRIPTION OF THE INVENTION
FIGS. 1A, 1B, 2A, 2B and 2C show a switch mode power supply 10 such
as may be used to provide DC power to a computer. Referring to FIG.
1A, AC power is received by power supply 10 at input terminals E1
and E2. After being filtered and rectified, a DC voltage appears at
+BULK. Switch S1 allows power supply 10 to be configured for either
110 or 220 volts AC input.
Referring to FIG. 1B, power supply 10 includes a controlled
switching circuit 12. A MOS field effect transistor Q1 is turned on
and off by a signal on the VOUT output (pin 6) of pulse width
modulator chip U1. As Q1 is turned on current flows from +BULK
through the primary winding of transformer T1 (pins 1,3). Switching
circuit 12 is powered by a tap off of transformer T1 at a secondary
winding (pins 4,5). The waveform at transformer T1 (pin 5) will be
a square wave which will be filtered to a DC voltage by capacitors
C10 and C11 and resistors R8 and R9. This voltage is applied to
chip U1 at the VIN input (pin 7). The signal pulse width at VOUT
(pin 6) will vary depending upon the magnitude of VIN. In a
preferred embodiment with the component values shown, VIN will be
regulated to about 13 volts. As VIN increases, the pulse width of
the VOUT signal decreases thereby reducing the duty cycle of Q1.
Similarly, as VIN decreases, the pulse width of the VOUT signal
increases to increase the duty cycle of Q1.
Referring now to FIG. 2A, an input 14 receives a power signal from
the secondary winding of transformer T1 (pins 10, 9). The power
signal at input 14 will be a square wave with a pulse width
controlled by pulse width modulator chip U1. According to a
preferred embodiment with the component values shown, the pulse
amplitude will be approximately 80 volts. A DC storage device in
the form of capacitor C17 is connected to input 14 through a diode
CR6-1. Capacitor C17 is connected between a node 16 and a reference
potential terminal B which in a preferred embodiment is ground.
When a power signal is received on input 14, the charge on
capacitor C17 will be approximately 17 volts.
A voltage regulator 18 is connected to capacitor C17 at node 16 and
has a primary output 20 which is connectable to a battery (not
shown). Voltage regulator 18 includes transistors Q2, Q3, Q4 and
Q5, resistors R16, R17, R18, R19, R20, R21 and R22, and capacitor
C18, and diodes CR7 and CR8. Resistor R16, transistor Q2 and diode
CR8 are connected in series between node 16 and output 20.
Resistors R19 and R18 and diode CR7 are connected in series between
a 5 volt auxiliary voltage source and the anode of diode CR8.
Resistor R17, transistor Q4 and resistor R20 are connected in
series between the control electrode of transistor Q2 and reference
potential terminal B. Resistors R21 and R22 are connected in series
between the anode of diode CR8 and reference potential terminal B.
The control electrode of transistor Q4 is connected to junction
between resistors R19 and R18, and capacitor C18 is connected
between the control electrode of transistor Q4 and the junction
between resistor R17 and transistor Q4. Transistor Q3 is connected
between node 16 and the control electrode of transistor Q2, and the
control electrode of transistor Q3 is connected to the junction
between resistor R16 and transistor Q2. Transistor Q5 is connected
between node 16 and the junction between transistor Q4 and resistor
R20, and the control electrode of transistor Q5 is connected the
junction between resistors R21 and R22.
In operation, voltage regulator 18 converts the voltage on
capacitor C17, which is nominally charged to 17 volts, to about
13.5 volts DC at output 20. The voltage at the anode of diode CR8
is about 14.5 volts and the voltage at the control electrode of
transistor Q5 is about 5 volts. The voltage at the control
electrode of transistor Q4 is also about 5 volts from the 5 volt DC
reference. The voltage on resistor R20 will about 4.4 volts which
keeps transistor Q4 on which in turn keeps transistor Q2 on. If the
voltage on the anode of diode CR8 exceeds 14.5 volts, the voltage
at the control electrode of Q5 will rise above 5 volts which will
force the voltage on resistor R20 above 4.4 volts which will turn
Q4 off. When transistor Q4 turns off, transistor Q2 turns off
thereby reducing the voltage on output 20.
Transistor Q3 operates as a current limiting switch. When the
current through R16 exceeds 250 ma, the voltage drop exceeds 0.6
volts which turns transistor Q3 on. This turns transistor Q2 off
thereby limiting current flow therethrough. In the event that the
battery is installed backwards (positive and negative terminals
reversed), resistor R18 and diode CR7 will pull the control
electrode of transistor Q4 to a negative voltage thereby turning
transistor Q4 off which turns transistor Q2 off. Capacitor C18 is
provided to compensate for small voltage swings in order to prevent
oscillation.
FIG. 2A also shows a diode CR6-2 connecting primary output 20 to
capacitor C17. As will be discussed more fully hereinafter, diode
CR6-2 not only prevents output 20 from being charged by the power
signal received on input 14, but in the event of an interruption of
AC power at input terminals E1 and E2, it allows capacitor C17 to
be charged directly from a battery attached to output 20.
Referring now to FIG. 2C, a DC to DC converter 22 is shown which
has an input 24 and output 26. Input 24 is connected to capacitor
C17 (See FIG. 2A). Converter 22 converts the DC voltage on
capacitor C17 to an auxiliary DC voltage at output 26. In a
preferred embodiment, DC to DC converter 22 is a down-switcher
which converts the voltage on capacitor C17 to 5 volts DC.
Converter 22 includes a pulse width modulator chip U2, transistor
Q10, inductor L2, resistors R32, R33, R34, R35, R36, R37, R38, R39,
R40, R41, R42, R43, R44 and R45, capacitors C22, C23, C24, C25, C26
and C27, and diode CR13. Transistor Q10, inductor L2 and resistor
R34 are connected in series between input 24 and output 26.
Resistor R32 is connected between the emitter and base of
transistor Q10, and resistor R33 is connected between the base of
transistor Q10 and the coupled C2-C1 output (pins 8 and 11) of U2.
Resistor R35 is connected between the junction of inductor L2 and
resistor R34 and the non-inverting NI2(+) input (pin 16) of U2.
Resistor R36 is connected between the non-inverting NI2(+) input
(pin 15) of U2 and reference potential terminal B. Resistor R37 is
connected between output 26 and the inverting I2(-) input (pin 15)
of U2. Capacitor C22, capacitor C23, resistor R38 and resistor R43
are connected in series between the I2(-) input (pin 15) of U2 and
reference potential terminal B. The junction between capacitors C22
and C23 is connected to the COMP output (pin 3) of U2, and the
junction between resistor R38 and resistor R43 is connected to the
inverting I1(-) input of U2. Capacitor C24 is connected between the
CT input (pin 5) of U2 and reference potential terminal B. Resistor
R39 is connected between the RT input (pin 6) of U2 and reference
potential terminal B. Diode CR13 is connected between reference
potential terminal B and the junction between transistor Q10 and
inductor L2. Resistor R40 and capacitor C25 are connected in
parallel between the VREF output (pin 14) and DTC input (pin 4) of
U2. Resistor R41 is connected between DTC input (pin 4) of U2 and
reference potential terminal B. Resistor R42 is connected between
VREF output (pin 14) and inverting I1(-) input (pin 2) of U2.
Resistors R44 and R45 are connected in series between output 26 and
reference potential terminal B. The junction between resistors R44
and R45 is connected to the noninverting NI1(+) input (pin 1) of
U2. Capacitors C26 and C27 are connected in parallel between output
26 and reference potential terminal B. The GND, OC, E1 and E2
connections (pins 7, 13, 9 and 10, respectively) of U2 are
connected to reference potential terminal B. Supply voltage VCC1 is
provided to supply voltage input VCC of chip U2 to energize DC to
DC converter 22. In a preferred embodiment, U2 is a TL494 pulse
width modulator chip.
In operation, pulse width modulator chip U2 provides pulses on
outputs C1, C2 in response to VCC1 supply voltage applied to U2 and
in response to other input signals received by chip U2 which vary
the pulse width. Outputs C1 and C2 are the collector electrodes of
two on chip transistors (their respective emitter electrodes E1 and
E2 are connected to reference potential terminal B). Pulses applied
to the control electrode of transistor Q10 allow the DC voltage on
capacitor C17 to be pulsed to bulk storage capacitor C26. Inductor
L2 and capacitor C26 filter the pulsating DC to provide an output
voltage of about 5 volts. Capacitor C27 provides high frequency
filtering to prevent transient voltage spikes. In order to regulate
the output, resistors R44 and R45 split the 5 volt output to about
2.5 volts which is input to U2 at pin 1. The VREF output of U2
supplies 5 volts which is split to 2.5 volts by resistors R42 and
R43. This 2.5 volt reference is input to U2 at pin 2. Pins 1 and 2
are noninverting and inverting inputs to an on chip op amp, the
output of which drives the on chip transistors with the C1 and C2
outputs. As the down-switcher output voltage increases above 5
volts, the NI1(+) input will increase above 2.5 volts. This will
reduce the pulse width of the signals output from C1 and C2 thereby
lowering the down-switcher output voltage. Similarly, if the output
voltage drops below 5 volts, the NI1(+) input will decrease below
2.5 volts, thereby increasing the pulse width of the signals on C1
and C2 and raising the down-switcher output voltage.
Resistors R34, R35, R36 and R37 provide overcurrent protection to
the down-switcher. The voltage at pin 15 is about 5 volts. When the
current through R34 reaches 1.8 amperes, the voltage on pin 16 will
exceed 5 volts. Pins 15 and 16 form the inverting and noninverting
inputs, respectively, to an on chip op amp, the output of which
drives the on chip transistors with the C1 and C2 outputs. As the
NI2(+) input increases above 5 volts, the pulse width of the
signals output from C1 and C2 are reduced thereby lowering the
current through resistor R34. Capacitors C22 and C23 and resistor
R38 provide compensation for the op amp inputs thereby improving
stability. The DTC input provides dead time control for the pulses
output by U2. At start up, the voltage on resistor R41 will be the
same as VREF or about 5 volts. This will maximize the dead time
between output pulses thereby preventing an overshoot condition. As
the voltage on capacitor C25 builds, the voltage on resistor R41
will approach that of the reference potential terminal B, zero
volts. This will provide the minimum dead time achievable by DTC.
Resistor R39 and capacitor C24 set the oscillation frequency of
U2.
FIG. 2B shows a switching circuit 28 for providing voltage from
capacitor C17 (see FIG. 2A) to supply voltage VCC1 input of
converter 22 (see FIG. 2C) during predetermined operating
conditions. Switching circuit 28 comprises three switches. The
first switch includes transistors Q6 and Q7 which are responsive to
the power signal on input 14 for providing capacitor C17 voltage to
VCC1. The second switch includes transistors Q6 and Q8 which are
responsive to the battery voltage at primary output 20 for
providing capacitor C17 voltage to VCC1. The third switch includes
transistor Q9 which is responsive to an auxiliary voltage on/off
signal received by input terminal 30 for overriding the second
switch. In addition to transistors Q6, Q7, Q8 and Q9, switching
circuit 28 includes resistors R23, R24, R25, R26, R27, R28, R29,
R30 R31 and R50, capacitors C20 and C21, diodes CR11 and CR22, and
zener diode CR12. Transistor Q6 is connected between capacitor C17
(FIG. 2A) More specifically, transistor Q6 is connected between
capacitor C17 and the cathode of diode CR22, the anode of diode
CR22 is connected to converter supply voltage input VCC1. Resistor
R25 and transistor Q7 are connected in series between the control
electrode of transistor Q6 and reference potential terminal B.
Resistor R24 is connected between capacitor C17 and the control
electrode of transistor Q6. In a preferred embodiment, transistor
Q6 is a PNP bipolar transistor with its emitter connected to
capacitor C17, its collector connected to VCC1 and its base
connected to resistors R24 and R25. Diode CR11, resistor R26, and
resistor R27 are connected in series between input 14 and the
control electrode of transistor Q7. In this manner, the control
electrode of transistor Q7 receives the power signal from input 14.
Capacitor C20 is connected between the junction of resistors R26
and R27 and reference potential terminal B. Resistor R28 is
connected between the control electrode of transistor Q7 and
reference potential terminal B. In a preferred embodiment,
transistor Q7 is an NPN bipolar transistor with its collector
connected to resistor R25, its base connected to resistors R27 and
R28, and its emitter connected to reference potential terminal
B.
Resistor R25 and transistor Q8 are connected in series between the
base of transistor Q6 and reference potential terminal B. Resistor
R23 and zener diode CR12 are connected in series between the
control electrode of transistor Q8 and primary output 20. Resistor
R29 is connected between the control electrode of transistor Q8 and
the reference potential terminal B. Thus, depending upon the
voltage on primary output 20 relative to the breakdown voltage of
zener diode CR12, the control electrode of transistor Q8 will
receive a signal from either primary output 20 or reference
potential terminal B. In a preferred embodiment, transistor Q8 is
an NPN bipolar transistor with its collector connected to resistor
R25 and the collector of transistor Q7, its emitter connected to
reference potential terminal B, and its base connected to the
junction between resistors R23 and R29. Also in a preferred
embodiment, resistor R23 is a variable resistor so that the zener
knee of zener diode CR12 may be adjusted for low current. Resistor
R50 is connected between the control electrode of transistor Q8 and
the collector electrode of transistor Q6.
Transistor Q9 is connected between the control electrode of
transistor Q8 and reference potential terminal B. Resistor R31 is
connected between input terminal 30 and the control electrode of
transistor Q9. In this manner, the control electrode of transistor
Q9 receives the on/off signal from input terminal 30. Capacitor C21
and resistor R30 are connected in parallel between the control
electrode of transistor Q9 and reference potential terminal B. In a
preferred embodiment, transistor Q9 is an enhancement type, N
channel MOS field effect transistor with its drain connected to the
base of transistor Q8, its source connected to reference potential
terminal B and its gate connected to resistors R30 and R31, and
capacitor C21.
In operation, when a power signal is received by input 14,
capacitor C17 will store a DC charge. In a preferred embodiment,
this will be about 17 volts. The power signal on input 14 will also
charge capacitor C20 through diode CR11 and resistor R26. The
voltage on capacitor C20 will bias NPN transistor Q7 on thereby
providing a connection between the base of transistor Q6 and
reference potential terminal B. With reference potential terminal B
at ground, PNP transistor Q6 will be biased on thereby providing
the voltage on capacitor C17 to VCC1. Pulse width modulator chip U2
of down-switcher 22 (see FIG. 2C) being powered up by this supply
voltage will convert the 17 volts of capacitor C17 appearing at
input 24 to 5 volts at output 26. Resistors R27 and R28 provide a
high resistance discharge path for capacitor C19 to reference
potential terminal B. It should be noted that as long as a power
signal appears on input 14, down-switcher 22 will provide an
auxiliary output and the states of transistors Q8 and Q9 are
irrelevant.
When a power signal is no longer received by input 14, capacitor
C20 will discharge through resistors R27 and R28. This will pull
the base of transistor Q7 low thereby shutting off transistor Q7.
However, if a battery has been properly connected to output 20 and
if its DC voltage exceeds the breakdown voltage of zener diode CR12
the voltage applied to the base of transistor Q8 will be sufficient
to turn transistor Q8 on. This provides a connection from reference
potential terminal B to the base of transistor Q6 which turns Q6
on, thereby providing the voltage on capacitor C17 to VCC1. It will
be noted that in a preferred embodiment, the DC voltage on primary
output 20 (also the connected battery) will be about 13.5 volts.
Thus, the voltage on capacitor C17 will be maintained at about 12.5
volts through diode CR6-2 (see FIG. 2A). 12.5 volts is a sufficient
supply voltage to power down-switcher 22 through VCC1.
Down-switcher 22 will also convert 12.5 volts appearing at its
input 24 to about 5 volts at output 26.
Assuming still that no power signal is received on input 14, there
are four conditions under which switching circuit 28 will
disconnect capacitor C17 from down-switcher 22. First, if no
battery has been connected to primary output 20, the base of
transistor Q8 will be pulled to reference potential terminal B
thereby shutting off transistor Q8 and consequently transistor
Q6.
Second, if a battery has been connected backwards to primary output
20, transistors Q8 and Q6 will be shut off. In this case, diode
CR22 protects pulse width modulator chip U2 (FIG. 2C). U2 has an
internal connection between its GND (pin 7) and VCC (pin 12). The
small current transistor Q6 conducts when shut off would be enough
to pull VCC to GND which would destroy U2. Diode CR22 effectively
prevents such leakage current.
Third, if a battery has been properly connected to primary output
20 but the battery has been discharged below a predetermined low
voltage limit which is below the breakdown voltage of zener diode
CR12, transistors Q8 and Q6 will be similarly shut off. It should
be noted that in the event that this predetermined limit is
reached, the leakage current through zener diode CR12 and resistors
R20 and R29 will be very small (less than 50 microamps). This will
produce a very small battery drain thereby protecting the battery
from any deep-discharge damage. Resistor R50 provides hysteresis
for the low battery cutoff voltage to prevent on/off oscillations.
This occurs because resistor R50 provides a path to reference
terminal B through chip U2. Thus, resistor R50 is connected in
parallel to resistor R29 when transistor Q6 is turned off. This
lower resistance effectively raises the turn on voltage of Q8
thereby preventing oscillations.
Fourth, a signal received on input terminal 30 can override the
second switch of transistors Q8 and Q9. For example, a high voltage
(off) signal on input 26 turns Q9 on which connects the base of
transistor Q8 to reference potential terminal B, thereby overriding
transistor Q8 and shutting transistors Q8 and Q6 off. A low voltage
(on) signal on input 26 turns Q9 off which allows transistor Q8 to
be controlled by the voltage on primary output 20. Resistors R31
and R30 and capacitor C21 provide noise suppression. The on/off
signal may be provided by any remote sensing device for controlling
the auxiliary voltage.
The following Table 1 provides preferred component values for
transistors Q1-Q10 according to the embodiment shown and described
herein.
TABLE 1 ______________________________________ TRANSISTOR VALUE
______________________________________ Q1 BUZ80A Q2 2N6107 Q3
2N3906 Q4 2N3904 Q5 2N3904 Q6 2N3906 Q7 2N3904 Q8 2N3904 Q9 BS170
Q10 TIP117 ______________________________________
The following Table 2 provide preferred component values for the
specified diodes according to the embodiment shown and described
herein.
TABLE 2 ______________________________________ DIODE VALUE
______________________________________ CR4 1N4946 CR5 1N4942 CR6
MUR620CT CR7 1N4148 CR8 1N4002 CR11 1N4148 CR13 5 A, 500 V CR22
1N4942 ______________________________________
It will be clear to those skilled in the art that the present
invention is not limited to the specific embodiment disclosed and
illustrated herein. It will be understood that the element values
shown in the drawings are illustrated by way of example only and
these values are not to be construed as limitations to the present
invention. Numerous modifications, variations, and full and partial
equivalents can be undertaken without departing from the invention
as limited only by the spirit and scope of the appended claims.
* * * * *